The invention relates to a semiconductor laser.
Semiconductor lasers, for example surface-emitting semiconductor lasers (VCSEL=vertical cavity surface emitting laser), are preferred light sources in the short-range area for optical data transmission in the data communications area over short and medium ranges of up to 10 km in length with high data transmission rates, the so-called bit rates. For the optical data transmission, the light emitted by the semiconductor lasers is coupled into optical fibres. In this case, the bit rate is limited principally by the time required for signal generation. By way of example, the direct current modulation of a VCSEL for generating an optical data signal limits the bit rate of the VCSEL to about 12.5 Gbit/s. The increase in electronic communication results in an increase in the requirements made of the optical data transmission and thus also the maximum achievable bit rate.
Higher bit rates of 40 Gbit/s, for example, are usually realized by means of external modulation or by means of wavelength bundling and are primarily used in optical data transmission for long ranges in excess of 10 km. In this case, high-power lasers are normally used to generate the optical signals.
A promising approach for increasing the bit rate over short and medium ranges, too, is wavelength bundling. In this case, usually a plurality of semiconductor lasers are used which each emit light at different wavelengths, each semiconductor laser emitting light of exactly one specific wavelength. In order to bundle the light signals emitted by the semiconductor lasers prior to coupling into an optical fibre, complicated integrated-optical or fibre-optic multiplexer elements have been necessary heretofore. However, these multiplexer elements impair the quality of the transmitted optical data signals on account of noise, scattering and absorption effects. Above all, a considerable additional outlay is necessary in order to carry out optical beam matching. Additionally, the electrical driving becomes more complicated and the chip area requirement rises.
The prior art discloses VCSELs in which either an active zone and a passive zone, cf. for example [1] to [4], or two active zones, cf. for example [5] to [8], are arranged adjacent to each other. The active zones serve for generating laser light and the passive zones serve as absorbers and thus as modulators or transmission switches. In the known VCSEL arrangements with a passive zone, an optical coupling of the two zones thus takes place in each VCSEL. Although laser light with two different wavelengths can be emitted in the case of the VCSELs with two active zones arranged one on the other, electronic-optical coupling of the two active zones always takes place. Consequently, the prior art does not disclose a VCSEL whose bit rate is higher than 12.5 Gbit/s.
The invention is therefore based on the problem of specifying a semiconductor laser in which the bit rate can be increased to above 12.5 Gbit/s in a simple manner and which can be produced simply and cost-effectively.
The problem is solved by means of a semiconductor laser having the features in accordance with the independent patent claim.
A semiconductor laser comprises a first reflector, a second reflector and a third reflector. A first optically active region is arranged between the first reflector and the second reflector, which can emit laser light of a first wavelength. Arranged between the second reflector and the third reflector is a second optically active region, which can emit laser light of a second wavelength, which is shorter than the first wavelength. The two optically active regions are essentially decoupled from one another both optically and electrically and can emit their laser light on a common optical axis in a common emission direction, the emission direction being directed along the optical axis from the first optically active region to the second optically active region.
One advantage of the invention can be seen in the fact that, on account of the optical and electrical decoupling of the two optically active regions, a semiconductor laser is created which can simultaneously generate two independent optical data signals in a simple manner, as a result of which the bit rate is increased by a factor of two. In this case, the skilful construction of the semiconductor laser has the effect that the laser light emitted by the first optically active region is transmitted essentially unimpeded through the second optically active region. The condition that the second wavelength is to be shorter than the first wavelength essentially avoids absorption of the laser light emitted by the first optically active region in the second optically active region. Consequently, the second optically active region does not perform the function of a transmission switch or modulator for the laser light emitted by the first optically active region. The semiconductor laser thus provides two independently controllable laser light sources with identical cross-sectional intensity distribution at two different wavelengths on an optical axis. This considerably reduces the chip area requirement before the input of an optical fibre.
A further advantage of the semiconductor laser is that the optical data signals emitted by the two optically active regions are emitted in a common emission direction on a common optical axis. The result of this is that the semiconductor laser can be optically coupled directly to an optical fibre for transmission of the emitted optical data signals, without requiring an optical component for wavelength bundling. Consequently, additional losses in such an optical component on account of noise, scatter and absorption effects cannot impair the emitted optical data signals. Thus, the optical data signals generated by the semiconductor laser can be coupled into the optical fibre with considerably reduced optical losses.
The semiconductor laser can be extended by further optically active regions and associated reflectors which are correspondingly stacked one on the other or arranged next to one another. Consequently, for the semiconductor laser, it is possible to achieve a further increase in the bit rate by an integer multiple in comparison with a semiconductor laser with a single optically active region. Clearly, in the case of an arrangement of n decoupled optically active regions, the bit rate can thus be increased by the factor n in comparison with just a single optically active region.
In a preferred embodiment of the semiconductor laser, Bragg reflectors are used as reflectors.
Preferably, in the semiconductor laser, an intermediate region is in each case provided between each optically active region and each adjacent Bragg reflector. This intermediate region serves for exactly setting the distance between two adjacent Bragg reflectors in order that the standing wave that forms between the two adjacent Bragg reflectors is set to the desired wavelength of the emitted laser light.
Preferably, at least one current confinement region is arranged in at least one intermediate region of the semiconductor laser adjacent to at least one of the two optically active regions. This current confinement region serves for controlling the current flow through the adjacent optically active region in such a way that the optical excitation in the optically active region is localized in a predetermined region. Thus, by way of example, the location of the optical excitation can be set to the optical axis.
The semiconductor laser is preferably designed as a surface-emitting semiconductor laser in which the Bragg reflectors and the active regions are arranged in stack form one above the other. This affords the considerable advantage that the Bragg reflectors and the active regions can be grown monolithically as a layer sequence in a multistage epitaxy method, in which case each stage of the epitaxy method can be correspondingly adapted in a simple manner depending on the layer to be produced. As an alternative, the semiconductor laser can also be designed as an edge-emitting semiconductor laser, for example as a DFB or DBR laser (DFB=distributed feedback, DBR=distributed Bragg reflector), in which the Bragg reflectors and the optically active regions are arranged next to one another.
In a preferred development of the semiconductor laser, the first Bragg reflector is set up in such a way that it comprises a reflectivity of xe2x89xa799%, preferably xe2x89xa799.7%, for the first wavelength and a reflectivity of xe2x89xa65% for the second wavelength. Furthermore, the second Bragg reflector is set up in such a way that it comprises a reflectivity of xe2x89xa798%, preferably 99.3%, for the first wavelength and a reflectivity of xe2x89xa799% preferably xe2x89xa799.7%, for the second wavelength. Moreover, the third Bragg reflector is set up in such a way that it comprises a reflectivity of xe2x89xa65% for the first wavelength and a reflectivity of xe2x89xa798%, preferably 99.3%, for the second wavelength.
This ensures that optical coupling of the two optically active regions of the semiconductor laser is essentially avoided. The feedback of the laser light of the first optically active region is effected by means of the first Bragg reflector and the second Bragg reflector, while the feedback of the laser light of the second optically active region is effected by means of the second Bragg reflector and the third Bragg reflector. There is thus no overlap of the feedback regions of the two optically active regions.
The first Bragg reflector and the second Bragg reflector are preferably at a distance from one another such that a Fabry-Perot resonance with a half-value width of up to 5 nm for the first wavelength can form between the first Bragg reflector and the second Bragg reflector. Correspondingly, the second Bragg reflector and the third Bragg reflector are preferably at a distance from one another such that a Fabry-Perot resonance with a half-value width of up to 5 nm for the second wavelength can form between the second Bragg reflector and the third Bragg reflector.
In a preferred development of the invention, at least one part of each Bragg reflector is designed as an electrical connection region for the respectively adjacent optically active region, in order to supply the two optically active regions with current. In this case, the electrical connection regions are preferably doped in such a way that the electrical connection regions of the first Bragg reflector and of the third Bragg reflector comprise excess charge carriers of a first charge carrier type and the electrical connection region of the second Bragg reflector comprises excess charge carriers of a second charge carrier type. This has the advantage that, on account of a suitable doping of the Bragg reflectors, the current paths through the two optically active regions can be decoupled from one another. Accordingly, a first electric current can be coupled into the first optical region by means of the first Bragg reflector and the second Bragg reflector and a second electric current can be coupled into the second optical region by means of the second Bragg reflector and the third Bragg reflector, independently of the first electric current.
To that end, the second Bragg reflector preferably comprises a first electrical connection region for the first optically active region and a second electrical connection regionxe2x80x94electrically insulated from the first electrical connection regionxe2x80x94for the second optically active region. As an alternative, the second Bragg reflector may also comprise a first Bragg partial reflector for the first optically active region and a second Bragg partial reflector for the second optically active region. In that case, however, the two Bragg partial reflectors in each case comprise an independent electrical connection region, the two electrical connection regions of the two Bragg partial reflectors being electrically insulated from one another.
An exemplary embodiment of the invention is illustrated in the figures and is explained in more detail below. In this case, identical reference symbols designate identical components.